Electrochemical corrosion is a process in which a metal atom oxidizes and loses electrons. In corrosion parlance, the location at which metal atoms lose electrons is called the anode and the location where the electrons are transferred is called the cathode. Localized corrosion within a base metal involves the creation of actively corroding anode areas separated from the non-corroding cathode areas. Thus, as the base metal oxidizes at the anode, forming a pit in the anodic area, the electrons left behind flow to the cathode. Additionally, at the metal surface, ions in the liquid phase accept electrons released by the metal. This flow of electrons and ions forms essentially an electrical circuit. As such, when the circuit is closed and an electrical potential difference exists between the anode and the cathode, an electrical current flows between the anode and the cathode.
A particular type of corrosion, molten salt induced corrosion or hot corrosion, can be a major problem in many high temperature applications. For example, condensing molten salt environments can exist in turbine engines, aircraft engines, micro turbines, and various industrial chemical processes. In a gas turbine, specifically, fuel impurities, such as sulfites, vanadium, or chloride, flowing in the hot gas path can interact at high temperatures to form molten salts on the surfaces of many of the hot gas path components within a gas turbine. For example, a molten salt layer may form on the surfaces of the buckets or nozzles in the turbine section of a gas turbine. This molten salt layer can dissolve away the metal oxide surface layer of such components, exposing the base metal to oxygen from the gases flowing in the hot gas path and, thereby, cause rapid oxidation of the base metal.
Currently, one of the only means for monitoring most industrial equipment exposed to molten salt corrosion is shut down and inspection of the equipment. For gas turbines, in particular, downtime can be very costly. Additionally, inspections often necessitate dismantlement of at least a portion of the gas turbine, which requires further downtime and expense. Other options may be available to a gas turbine operator, such as fuel quality monitoring and filter maintenance, but it is difficult to predict when such actions need to be taken. As such, unnecessary expenditures are made when any of these actions are performed too frequently or too infrequently.
Accordingly, a system and method for online monitoring of molten salt corrosion of high temperature components would be welcome in the technology.